Microwave resonators are known for use in time and frequency standards, frequency stable elements, as well as building blocks for passive devices such as filters and the like. The performance of the microwave resonator is gauged by its Q-value, expressed as EQU Q=2.pi. f.sub.0 * (Storage energy/Loss power), (1)
where f.sub.0 is the resonant frequency of the microwave resonator. (See Hayt, J. R., "Engineering Electromagnetics", 1981, p. 472). As shown in Equation (1), the Q-value of the microwave resonator can be increased by reducing the loss power associated with factors such as conductor loss, dielectric loss, and radiation loss.
Low temperature (T.sub.c), such as 4K, superconducting microwave resonators which employ a superconducting cavity made of Nb are known to have Q-values from about 10.sup.6 to 10.sup.9. (See V. B. Bragrinskii, et al: "The Properties of Superconducting Resonators on Sapphire", IEEE Trans. on Magn. Vol. 17, No. 1, P955, 1981, as a reference.) Although low T.sub.c Nb microwave resonators have high Q-values, they must operate at very low temperatures (below 9K). These microwave resonators require use of curved cavity walls. Curved cavity walls of materials which have a high T.sub.c, of for example 77K, however, are difficult to produce. On the other hand, high Q-value microwave resonators formed merely from a dielectric without an associated conducting medium also have high Q-values (see D. G. Blair, et al: "High Q Microwave Properties of a Sapphire Ring Resonator", J. Phys. D: Appl. Phys., 15, P1651, 1982.) However, the problems associated with the far reaching evanescent fields make them very bulky and vulnerable to microphonic effect, which limits the applications.
The need therefore exists for microwave resonator made of high T.sub.c, such as 77K, superconductor that have Q-values comparable to low T.sub.c superconducting microwave resonators made of Nb.